The invention concerns steam dealkylation reactions for manufacturing benzene or its lower homologs by dealkylation of toluene or other alkylbenzene hydrocarbons by means of a catalyst containing a carrier, for example, an alumina carrier, and one or more noble metals of the platinum family associated, on the one hand, to at least one other metal or derivative, said metal being selected from rhenium, gold, silver, copper, titanium and, on the other hand, to at least one alkali metal selected from lithium, sodium, potassium, rubidium and cesium. These catalysts further contain halogen as specified below; the invention has particularly for object a process for regenerating the used catalyst.
In the technical literature, many publications indicate that catalysts may be regenerated, particularly those containing one or more metals of the platinum family, by mere combustion of the carbonaceous deposits contained therein; however, said disclosed processes are not finally profitable on an industrial scale, since it has been observed that such a regeneration system does not restore the initial activity of the catalyst and that, after each regeneration, the activity of the catalyst decreases more and more rapidly. In addition, it has also been observed that the selectivity of the so-regenerated catalysts decreases also very rapidly after a relatively short period of operation.
It is an object of the present invention to provide an improved method for regenerating a catalyst used in catalytic steam dealkylation to produce benzene, toluene, xylenes, ethylbenzenes and substantial amounts of hydrogen. The dealkylation may also be performed, for example, to dealkylate toluene, xylenes, ethylbenzene, propylbenzene, methylbenzene or hydrocarbons with condensed rings such as naphthalene, phenanthrene, anthracene etc . . . , or to dealkylate mesitylene, pseudo cumene, hemimellitene; it is also possible to subject to an aromatization followed with a dealkylation such hydrocarbons as alkylcyclohexane, alkyltetralin, alkyldecalin and alkyldihydroanthracene. Nitrogen containing aromatic compounds, such for example as pyridine derivatives, may also be dealkylated, the nitrogen being eliminated as NH.sub.3 or N.sub.2.
The steam dealkylation is generally performed in the presence of catalysts, between 300.degree. and 600.degree. C., preferably between 350.degree. and 550.degree. C., under a pressure from 1 to 20 atmospheres and preferably from 3 to 10 atmospheres, with a LHSV ("Liquid Hourly Space Velocity") i.e. liquid VVH (space velocity) from 0.1 to 10 volumes of hydrocarbons per catalyst volume and per hour, preferably from 1 to 5, with a ratio (by moles) H.sub.2 O/hydrocarbons from 1 to 20, preferably from 3 to 15.
These operating conditions are particularly efficient for dealkylating alkyl aromatic hydrocarbons obtained in the reactions of catalytic reforming or for producing aromatic hydrocarbons ("Aromizing").
The process of this invention for catalyst regeneration thus concerns the process for regenerating and restoring a specific catalyst, so that the reaction system may be operated in a continuous manner over a time period much longer than according to the prior art and without substantial decrease of the activity and the selectivity of the catalyst, the latter substantially recovering, after each regeneration and rejuvenation period, its initial activity and also its initial selectivity.
The regeneration process is particularly applicable to the regeneration of very specific catalysts used for obtaining dealkylated aromatics with high yields (for example high benzene yields) simultaneously with a low rate of degradation of the aromatic ring and giving a reaction gas of high hydrogen content (from about 50 to about 70% by volume of hydrogen) of substantial value.
These specific catalysts contain a carrier, preferably alumina, and from 0.1 to 1%, preferably 0.2 to 0.8% and more particularly 0.25 to 0.65%, by weight with respect to the catalyst, of at least one noble metal of the platinum family, particularly rhodium. They contain 0.5 to 4% and preferably 0.8 to 2% by weight, with respect to the catalyst, of halogen (preferably chlorine).
They also contain either 0.05 to 1%, preferably 0.06 to 0.5% and more preferably, 0.07 to 0.3% of titanium oxide, of formula TiO.sub.2, expressed by weight with respect to the catalyst, or 0.05 to 2% of rhenium or of another metal selected from gold, silver and copper.
These catalysts further contain 0.01 to 5% by weight, with respect to the catalyst, of at least one alkali metal selected from lithium, sodium, potassium, rubidium and cesium and optionally 0.01 to 6% by weight, with respect to the catalyst, of at least one additional metal or compound of an additional metal selected from indium, zirconium, thorium, germanium, tin, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, iron, cobalt and nickel.
Catalysts of a preferred type contain, for example:
(a) an alumina carrier of specific surface higher than 50 m.sup.2 per gram and, preferably, higher than 80 m.sup.2 per gram, and, by weight: PA0 (b) from 0.1 to 0.5% of rhodium and 0.1 to 0.5% of at least one noble metal selected from the group consisting of platinum, palladium and ruthenium, PA0 (c) from 0.1 to 1% of rhenium, PA0 (d) from 0.5 to 5% of at least one alkali metal selected from the group consisting of lithium, sodium, potassium, rubidium and cesium. PA0 first, in a first step, the inert gas used to scavenge the pretreated catalyst as above mentioned, is progressively replaced with a gas containing molecular oxygen (step c); the oxygen partial pressure is at least 0.005 bar and, preferably, from about 0.02 to 0.14 bar, the temperature of the gas at the inlet of the disconnected reaction zone being comprised between about 300.degree. and 550.degree. C. and preferably between about 350.degree. and 450.degree. C.
The catalyst carrier is preferably selected from eta-cubic .eta..sub.C, gamma-cubic .gamma..sub.C, gamma tetragonal .gamma..sub.T, chi cubic .alpha..sub.C, kappaorthorhombic K, theta monoclinic .theta., delta orthorhombic .delta. and rho amorphous .rho. aluminas.
Preferably, before any contact with the alkylaromatic hydrocarbons to be dealkylated, the catalyst is subjected to a preliminary reduction treatment by passage of a hydrogen stream at a temperature from 100.degree. to 500.degree. C.
The regeneration process of the invention requires, for the dealkylation reaction, the use of at least two reactors containing the catalyst, one of these reactors being in operation and the second being subsequently disconnected from the circuit and submitted to the regeneration step. Once the regeneration is terminated, the second reactor is put in service while at least one of the ther reactors, if necessary, is in turn disconnected from the circuit to proceed to the regeneration of the catalyst contained therein and so on, so that the dealkylation reaction can be effected in a continuous manner. The reactors in operation are connected in parallel or in series according to the needs.
Various furnaces are arranged at adequate locations to preheat the charge and the intermediary effluents before introducing them into the operating reactors. For example in the case of two operating reactors, arranged in series, a furnace is provided at the inlet of the first reactor, a second furance being located between the first and the second reactors, etc . . . Simultaneously in a third reactor, there is carried out the regeneration and the rejuvenation of the catalyst contained therein.
There can be designed a system of three reactors wherein the two first reactors, operated in series, may be operated over a very long period of about 300 hours or more and, according to the nature of the charge, the one or the other of these two reactors will be, at a convenient time, disconnected while the third reactor will be put in service and take the place, in the path of the charge , of that one of the two reactors which has been disconnected.
The invention concerns a process for regenerating the dealkylation catalyst and more particularly it concerns a pretreatment of the catalyst before the regeneration treatment itself of this catalyst. The pretreatment as well as the treatment of the deactivated catalyst are performed in the reaction zone which has been disconnected, precisely to proceed to the regeneration of the catalyst contained therein. The regeneration process thus comprises seven steps (a) to (g).
Thus, before the regeneration step itself, the catalyst is first subjected to a hydrogen treatment (step a). This treatment consists of an "elution" or "washing" or "scavenging" of the catalyst with hydrogen. This treatment is performed by passing a hydrogen stream through a bed of the deactivated catalyst. To perform this "washing" the temperature of the catalyst or of the disconnected reaction zone, is maintained between about 300.degree. and 700.degree. C., the pressure in the reaction zone (i.e. in this case the disconnected reaction zone containing the catalyst to be regenerated) being from about 1 to 25 bars, the space velocity being from about 1 to 10,000 h.sup.-1 (volumes/catalyst volume per hour) and preferably from about 10 to 5,000 h.sup.-1. Preferably, during the washing with hydrogen, the temperature is progressively decreased from the temperature level of the catalyst bed when the catalytic reaction was discontinued, down to 300.degree. C. Optionally, according to a preferred method, the "elution" or "washing" of the catalyst is performed by means of a hydrogen stream containing halogen or a halogenated compound selected from chlorine, bromine, fluorine and derivatives of said halogens. Preferably, chlorine or a chlorinated compound is used; in the operating conditions at which is performed the hydrogen treatment, the halogen (or the halogen compound optionally present in the hydrogen stream) either is maintained in the form of Cl.sub.2 or Br.sub.2, or is converted to a hydrogen halide (HCl, HBr, HF) and thus, either in the form of Cl.sub.2 or Br.sub.2 or as halogenated acid, is fixed on the catalyst so as to compensate the halogen loss of the catalyst occuring during the dealkylation reaction. The amount of halogen or halogenated compound used in the hydrogen stream is so selected that after "elution" or "washing" with hydrogen, the catalyst contains by weight 0.5 to 4% and preferably 0.8 to 2% of halogen with respect to the catalyst. Thus, the content of halogen or halogenated compound in the hydrogen stream is (expressed as halogen) comprised between 0.001 and 10 moles of halogen per 100 moles of hydrogen and preferably from about 0.05 to 2 moles of halogen per 100 moles of hydrogen.
After termination of this pretreatment, the catalyst is subjected to a scavenging with an inert stream (nitrogen for example) to remove residual hydrogen from the disconnected reaction zone containing the catalyst to be regenerated (step b).
Then the catalyst is subjected to the regeneration treatment itself. This treatment, preferably, is conducted as follows:
Then in step (d), the gas containing molecular oxygen is passed over the catalyst, said gas being introduced at a temperature from 300.degree. to 550.degree. C. under a pressure of from about 1 to 25 bars so as to remove the coke deposited on the catalyst. The space velocity is from 1 to 500 volumes/catalyst volume per hour. The gas used to produce the combustion of the coke may optionally contain 0.01 to 10% by volume of steam. It may optionally contain a halogen or a halogen comound, in a sufficient amount (for example from 0.01 to 10 moles of halogen and preferably from 0.05 to 2 moles) to avoid a too substantial loss of the halogen contained in the catalyst. The gas used may optionally contain simultaneously steam and a halogen or a halogen compound.
The formation of a combustion front (or ignition zone) which slowly passes through the catalyst bed is observed, during the treatment of the catalyst by means of the molecular oxygen containing gas. The oxygen content of the gas is so regulated as to prevent the temperature of said combustion front from exceeding about 650.degree. C. Thus, this front is maintained preferably at a temperature which does not exceed by more than about 150.degree. C. the temperature of the gas at the inlet of the catalyst bed. As the combustion front progresses through the catalyst bed, the inlet side of the reactor containing said catalyst bed cools down, but, on the contrary, on the outlet side of the catalyst bed, the temperature progressively increases as a result of the heat evolution produced by the combustion of the carbonaceous materials on the catalyst. When this exothermic effect is no longer observed, the calcination in pure air (or an equivalent gas) may be continued at a temperature from about 300.degree. to 500.degree. C. for a sufficient time, so as to reduce the carbon content of the catalyst by at least 90% of its initial content before regeneration, i.e. at the time of the disconnection of the reaction zone containing the used catalyst.
Then, the combustion of the carbonaceous material being complete, the catalyst is reactivated (step e), by maintaining it in a stream of air or an equivalent gas (space velocity from about 1 to 10,000 and preferably 100 to 5,000 volumes/catalyst volume per hour at a temperature from about 300.degree. to 500.degree. C., preferably in the presence of at least one halogen or halogenated compound in a sufficient amount to proceed to an optimum reactivation of the active species, i.e. in a sufficient amount to bring the halogen content of the catalyst to its optimum value once the reactivation is terminated. This air-halogen or air-halogen compound mixture may optionally contain 0.01 to 10% by volume of steam. The pressure in the catalyst zone during this operation is maintained between about 1 and 25 bars. The catalyst is maintained in contact with the halogen or halogen derivative containing gas for a sufficient time, so that the catalyst, ready for use, contains 0.5 to 4% by weight, preferably 0.8 to 2% by weight of halogen with respect to the dry catalyst mass. The preferred halogen is chlorine. In order to adjust the halogen content of the catalyst, the halogen may be used simply in its elemental form, but in order to facilitate the handling, it is preferred to use halogen derivatives (preferably chlorine derivatives) which are normally in the liquid state, provided however that these compounds may release their halogen when they are in contact with the catalyst, under the indicated operating conditions.
The preferred chlorinated derivatives which can be normally used are chlorinated hydrocarbons having from 1 to 4 carbon atoms per molecule, for example carbon tetrachloride, chloro-ethylene, dichloroethylene, tert. butyl chloride, etc . . .
Generally, in order to avoid any corrosion, it is preferred to discontinue the halogen treatment as soon as it is observed that the halogen content of the gas issued from the regeneration zone is close to the halogen content of the gas supplied to said regeneration zone.
Then, after this halogenation phase, the catalyst is subjected to a calcination step (step f), in the presence of air or of an equivalent gas, at a temperature from about 300.degree. to 600.degree. C., preferably from 400.degree. to 500.degree. C. The air flow rate is such that the space velocity is from 1 to 10,000, and preferably from 100 to 5,000 volumes/catalyst volume per hour. The pressure is comprised between about the atmospheric pressure and 25 bars. The duration of the calcination step is from about 15 minutes to 180 minutes.
Finally, after said calcination, the reactor and the catalyst contained therein are purged (step g), by passing a stream of inert gas, for example nitrogen or another gas, so as to remove any oxygen trace.
Said purge may be performed, for example, under the same operating conditions as those used in the preceding calcination step.
Optionally, after said purge with an inert gas, the catalyst is advantageously reduced by means of a hydrogen containing gas and for example by means of a gas produced by the unit itself, before connecting again the reactor containing the regenerated and reactivated catalyst to the dealkylation unit.
This reduction is thus conducted in the presence of a gas containing at least 10% (by mole) of hydrogen, preferably as dry gas, i.e. containing less than 0.5% by weight of water; the reduction temperature is comprised between about 200.degree. and 700.degree. C. and preferably between about 325.degree. and 550.degree. C.; the total pressure in the reactor is comprised between the atmospheric pressure and about 25 bars. The space velocity of hydrogen is from about 1 to 10,000 volumes of hydrogen per catalyst volume and per hour and preferably from 10 to 5,000. The reduction time is preferably not shorter than 1 hour without exceeding about 30 hours.
The process of the invention thus provides during the regeneration of the catalyst, for a clear improvement of the activity and selectivity as compared to the conventional regeneration methods which do not include the initial washing or elution with a hydrogen stream.